1. Field of the Invention
The present invention relates to a displacement measuring apparatus that measures a relative displacement between a structure and a sensor.
2. Description of the Related Art
Previously, there has been a displacement measuring apparatus that illuminates light from a light source onto a surface of a structure so as to measure a displacement of the structure using a light beam reflected on the surface. The light beam reflected on the surface of the structure enters a light receiving element while generating a light intensity distribution in the reflected light beams in accordance with a surface roughness or a reflectance distribution of the structure. Since the light intensity distribution of the reflected light beams changes in accordance with the movement of the structure, the displacement of the structure can be measured by detecting the change of the light intensity distribution using the light receiving element. Japanese Patent Laid-Open No. 2003-166855 discloses a displacement measuring apparatus that is provided with a scale on the surface of the structure in which a reflectance is periodically changed in order to improve measurement accuracy of the displacement of the structure.
However, in the displacement measuring apparatus disclosed in Japanese Patent Laid-Open No. 2003-166855, when the reflectance of the surface of the structure is low, a signal amplitude that is detected by a sensor is lowered and therefore a signal-to-noise ratio (S/N) of the signal amplitude detected by the light receiving element is deteriorated. Accordingly, signal accuracy with respect to position information is lowered, and position detection accuracy is deteriorated. On the other hand, when an output of illumination light from the light source is increased in order to obtain reflected light intensity, a consumption of current of the light source is increased and a lifetime and a reliability of the light source are affected.
The signal amplitude detected by the sensor is maximized when a reflection surface and a scattering surface (non-reflection surface) have the same lengths each other in a period direction of the scale. Therefore, the length of the reflection surface cannot be wider than or equal to the same length as that of the scattering surface. In addition, a reflectance of the scale depends on a surface reflectance of the structure, and a material of the structure needs to be changed in order to obtain a desired reflectance. However, it is difficult to change the material since an original function of the structure may be lost by the change of the material.